In general, in a process of producing a rail for railroad, a steel material is heated and, after hot-rolled into a certain shape at the austenite region temperature or higher or after reheated to the austenite region temperature or higher, the resulting steel is forcedly cooled to secure a desired quality such as hardness required for a rail head. This forced cooling is performed by jetting a cooling medium (air, water, mist, or the like) to a rail until the temperature of the rail head reaches 350° C. to 450° C. while controlling the temperature history, whereby the rail head can have a fine pearlite structure and thus the rail can have a high hardness with improved wear resistance and toughness. For example, under a severe use environment for a rail, like railroad transportation in a mine of natural resources such as coal, in which the loading weight is heavier than that of a passenger car, for example, the rail severely wears, and the life-span for using the rail is short. Accordingly, wear resistance and toughness thereof are particularly required to be improved.
Bainite has low wear resistance, and martensite has low toughness. Accordingly, to achieve high wear resistance and high toughness simultaneously, bainite transformation and martensite transformation of the rail head that occur during the above-described forced cooling are required to be prevented for the rail head in order to have a pearlite structure stably. In addition, because pearlite has higher wear resistance and higher toughness with smaller lamellar spacing, it is important to achieve finer lamellar spacing.
The transformation to bainite or martensite during the forced cooling is affected by a cooling rate during the forced cooling. In particular, if the cooling rate is 3° C./s or higher all the time during the forced cooling, it is highly possible that the transformation to bainite or martensite occurs. As a technique to solve this type of problem, Patent Literature 1, for example, discloses a technique in which the rate of cooling a head surface until pearlite transformation starts is set to 1° C./s to 10° C./s, and the rate of cooling the head surface until the pearlite transformation in a region at 20 millimeters or deeper from the surface ends is set to 2° C./s to 20° C./s. Patent Literature 2 discloses a technique of suppressing tempering of pearlite. This suppression is accomplished by performing first forced cooling from a temperature range of 750° C. or higher down to 600° C. to 450° C. at a cooling rate of 4° C./s to 15° C./s and then, after temporary stop of the forced cooling to raise the temperature thereby ending pearlite transformation, performing second forced cooling down to 400° C. at a cooling rate of 0.5° C./s to 2.0° C./s.